Add a nop instruction if a section starts with landing pad for functi…

…on splitter This change adds a nop instruction if section starts with landing pad. This change is like [D73739](https://reviews.llvm.org/D73739) which avoids zero offset landing pad in basic block sections. Detailed description: The current machine functions splitter can create ˜sections which start with a landing pad themselves. This places landing pad at offset zero from LPStart. ``` .section .text.split.foo10,"ax",@progbits foo10.cold: # %lpad .cfi_startproc .cfi_personality 3, __gxx_personality_v0 .cfi_lsda 3, .Lexception5 .cfi_def_cfa %rsp, 16 .Ltmp11: <--- This is a Landing pad and also LP Start as it is start of this section movq %rax, %rdi <--- first instruction is at offest 0 from LPStart callq _Unwind_Resume@PLT ``` This will cause landing pad entries to become zero (.Ltmp11-foo10.cold) ``` .Lcst_begin4: .uleb128 .Ltmp9-.Lfunc_begin2 # >> Call Site 1 << .uleb128 .Ltmp10-.Ltmp9 # Call between .Ltmp9 and .Ltmp10 .uleb128 .Ltmp11-foo10.cold <---This is zero # jumps to .Ltmp11 .byte 3 # On action: 2 .uleb128 .Ltmp10-.Lfunc_begin2 # >> Call Site 2 << .uleb128 .Lfunc_end9-.Ltmp10 # Call between .Ltmp10 and .Lfunc_end9 .byte 0 # has no landing pad .byte 0 # On action: cleanup .p2align 2 ``` The C++ ABI somehow assumes that no landing pads point directly to LPStart (which works in the normal case since the function begin is never a landing pad), and uses LP.offset = 0 to specify no landing pad. This change adds a nop instruction at start of such sections so that such a case could be avoided. Output: ``` .section .text.split.foo10,"ax",@progbits foo10.cold: # %lpad .cfi_startproc .cfi_personality 3, __gxx_personality_v0 .cfi_lsda 3, .Lexception5 .cfi_def_cfa %rsp, 16 nop <--- new instruction that is added .Ltmp11: movq %rax, %rdi callq _Unwind_Resume@PLT ``` Reviewed By: modimo, snehasish, rahmanl Differential Revision: https://reviews.llvm.org/D130133
llvm · Jul 22, 2022 · 3bb1ce2 · 3bb1ce2
1 parent 2ebfda2
commit 3bb1ce2
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Showing 4 changed files with 27 additions and 5 deletions.
diff --git a/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h b/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h
@@ -25,6 +25,8 @@ using MachineBasicBlockComparator =
 void sortBasicBlocksAndUpdateBranches(MachineFunction &MF,
                                       MachineBasicBlockComparator MBBCmp);
 
+void avoidZeroOffsetLandingPad(MachineFunction &MF);
+
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_BASICBLOCKSECTIONUTILS_H
diff --git a/llvm/lib/CodeGen/BasicBlockSections.cpp b/llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -268,8 +268,8 @@ void llvm::sortBasicBlocksAndUpdateBranches(
 // If the exception section begins with a landing pad, that landing pad will
 // assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
 // zero implies "no landing pad." This function inserts a NOP just before the EH
-// pad label to ensure a nonzero offset. Returns true if padding is not needed.
-static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
+// pad label to ensure a nonzero offset.
+void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
   for (auto &MBB : MF) {
     if (MBB.isBeginSection() && MBB.isEHPad()) {
       MachineBasicBlock::iterator MI = MBB.begin();
@@ -278,10 +278,8 @@ static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
       MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
       BuildMI(MBB, MI, DebugLoc(),
               MF.getSubtarget().getInstrInfo()->get(Nop.getOpcode()));
-      return false;
     }
   }
-  return true;
 }
 
 // This checks if the source of this function has drifted since this binary was

diff --git a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
@@ -146,7 +146,7 @@ bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
     return X.getSectionID().Type < Y.getSectionID().Type;
   };
   llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator);
-
+  llvm::avoidZeroOffsetLandingPad(MF);
   return true;
 }
 

diff --git a/llvm/test/CodeGen/X86/machine-function-splitter.ll b/llvm/test/CodeGen/X86/machine-function-splitter.ll
@@ -242,6 +242,28 @@ define void @foo9(i1 zeroext %0) nounwind #0 !prof !14 {
   ret void
 }
 
+define i32 @foo10(i1 zeroext %0) personality ptr @__gxx_personality_v0 !prof !14 {
+;; Check that nop is inserted just before the EH pad if it's beginning a section.
+; MFS-DEFAULTS-LABEL: foo10
+; MFS-DEFAULTS-LABEL: callq   baz
+; MFS-DEFAULTS:       .section        .text.split.foo10,"ax",@progbits
+; MFS-DEFAULTS-NEXT:  foo10.cold:
+; MFS-DEFAULTS:       nop
+; MFS-DEFAULTS:       callq   _Unwind_Resume@PLT
+entry:
+  invoke void @_Z1fv()
+          to label %try.cont unwind label %lpad, !prof !17
+
+lpad:
+  %1 = landingpad { ptr, i32 }
+          cleanup
+          catch ptr @_ZTIi
+  resume { ptr, i32 } %1
+
+try.cont:
+  %2 = call i32 @baz()
+  ret i32 %2
+}
 
 declare i32 @bar()
 declare i32 @baz()